This invention relates to microwave ovenable trays and packages which are constructed so as to produce browning and/or crisping of a food product or products being heated, and more generally to microwave ovenable trays and packages that provide localized or concentrated heating effects for the food contained therein.
A variety of food products do not cook satisfactorily when heated by microwave energy alone. Although the food may be heated to the required temperature, the texture and/or color may not be at all comparable to that achieved when the food is cooked in a conventional oven. Effects that are not sufficiently achieved by microwave heating alone include surface crispness and surface browning for products with a doughy crust such as pizza and breaded products such as fish cakes. Also a surface singeing effect that is often desired in the cooking of meat is not effectively achieved by microwave heating alone. Microwave oven manufacturers have provided ovens with special features that at least partially overcome the shortcomings of microwave ovens just described, but these features, which include browning elements and hot-air convection heating are not available on all ovens and they add substantially to the cost of the ovens that are so equipped. Another approach that has been developed and promoted mainly by the manufacturers of ovenware dishes and utensils is the use of ceramic dishes or trays that have a component, such as a coating on the underside of the dish, that absorbs microwave energy and converts it to thermal energy that is transmitted to the food to provide the desired crisping, browning and/or singeing effect. Although these special dishes and trays do provide the desired effects they are expensive and the use of such a dish will in many cases contradict the convenience aspect of microwave cooking, particularly when the food product is an item such as frozen pizza that the consumer would expect to be able to use without involving a dish or tray that must later be cleaned.
It is highly desirable to package foods, and particularly foods that are prepared to save the consumer time and effort, in packages that serve in some manner as the container for cooking the food. It is further desireable that the package be designed and constructed so that the food will have both the consistency or appearance, as well as the temperature uniformity, that the consumer expects. Thus, packages for food intended for microwave cooking should incorporate features that provide for localized and concentrated heating effects, such as browning and crisping, where these are desireable for a particular food product.
The prior art provides a number of package constructions that are intended to provide special heating effects such as those discussed above during microwave cooking of the food contained in the package. For example, U.S. Pat. No. 4,267,420 issued to Brastad shows a wrapping material, formed of polyester film of a thickness of approximately 0.0005 to 0.001 inches. A thin coating is applied to the film, the coating typically being aluminum which has been evaporated onto the film. The action is such that when the wrapping substance is placed around a food product, a portion of the microwave energy is converted to heat by the evaporated aluminum coating. This heat is transferred to the food item being heated and the food item is thereby browned or crispened. A portion of the microwave energy which is not converted to heat by the aluminum coating passes through the wrapping to thereby heat the food item by the direct action of microwave energy.
U.S. Pat. No. 4,555,605 issued to Brown et al is another example of a food package or food container which is constructed so as to provide browning or crisping of a food product to thereby enhance its consistency and appearance immediately prior to consumption. In the Brown et al construction a microwave interactive layer, formed of a metallic coating for example, is placed on one surface of a food support member. The browning action is substantially the same as that of the Brastad construction.
While presumably at least partially successful in effecting browning and crisping of a food product, both of these prior art constructions carry with them a significant disadvantage. In both the Brastad construction and the Brown et al construction, the microwave interactive component, which is typically a plastic film carrying a thin metallic coating, is in direct contact with the food item to be browned or crispened. This creates a risk, under certain use conditions, that the food may be contaminated because of degradation of the microwave interactive component. For example, it is not unlikely that a package will be subjected to a longer cooking time than intended or placed in a higher wattage microwave oven than intended. Under such conditions the microwave interactive component can overheat to such a degree that it becomes embrittled and possibly cracks, while at the same time the food burns and becomes stuck to the degraded material. Thus, there is a chance that pieces of the microwave interactive material will stick to the food and be ingested by the consumer.
A microwave oven tray or package which does not exhibit this disadvantage is shown in U.S. Pat. No. 4,190,757, issued to Turpin et al. In that construction a microwave absorptive lossy substance (46) is on the underside of the food contacting layer (44) with the food contacting layer formed of aluminum, steel, copper, brass, ceramic foil, sheet mica, Portland cement, plaster of Paris, or a non-metallic mineral or a thin glaze of ceramic. Thus, in the Turpin et al construction, there can be no contacting of the food being cooked and browned by the microwave absorptive lossy substance, because of the presence of the metal or ceramic, etc. layer. The materials employed are of relatively high heat-resistance and thermal conductivity. These materials exhibit great resistance to burning, charring, discoloration, smoking, or the emission of odors. These materials are also further suitable for the purpose intended because each provides a suitable medium for the transmission of heat from the layer of microwave absorptive lossy material to the food. However, these materials have certain definite disadvantages that limit their usefulness in microwave food packaging applications. The metals aluminum, steel, copper and brass all are microwave opaque and will thus block the microwave energy from reaching the absorptive lossy material from one direction. Thus when such metallic material is used as the food contacting layer, the functionality of the package may be limited because the patterns of energy distribution differ markedly among different microwave ovens and the amount of energy reaching the absorptive lossy material will vary from oven to oven. Also, the use of metals such as aluminum in packages intended for use in microwave ovens creates a possibility for arcing if the metal in the package touches another metal item in a microwave oven such as the wall of the oven, a metal rack or another package of the same type. The other materials proposed by Turpin et al for the food contacting layer, ceramic foil, sheet mica, Portland cement, plaster of Paris, etc. are not suitable for use in disposible packaging materials because they lack the characteristics of formability or bendability that are important in the manufacture of inexpensive packages.
Another example of a prior art construction wherein a tray is modified to produce a browning effect for microwave cooking is afforded by Canadian Pat. No. 1,153,069 issued Aug. 30, 1983. In that construction, an upper or food-contacting plastic film (14) is provided on its lower surface with a vacuum deposited metallic layer (16) which, due to the thinness of the metallic layer, exhibits the required characteristic of converting microwave energy to heat. Beneath the energy-converting layer (16) is a layer of stock material (18) which may be formed from paper, glassine materials, plastics, ceramics, and various coated papers. Again, as with the above-noted patent to Brown et al, this construction suffers the disadvantage of possible cracking of the food contacting plastics layer, due to excessive heating of it by the energy-converting layer.